The invention relates to a circuit configuration for generating a controllable output voltage having a voltage generator and a comparator, which, on an output side, is connected to a control terminal of the voltage generator.
Voltage generators for generating voltage levels that deviate from the supply voltage within an integrated circuit are necessary in many cases. In integrated volatile semiconductor memories, so-called dynamic random access memories (DRAMs), for example, a negative voltage is generated which negatively biases the semiconductor substrate with respect to the applied supply voltage. The positive supply voltage is fed to the voltage generator, the latter generating the negative substrate bias voltage from the supply voltage.
During test operation, in particular, it is necessary for the semiconductor chip to be operated at different substrate bias voltages. Thus, during test operation, it is desirable that the substrate bias voltage generator can be switched off, on the one hand, and, on the other hand, can be trimmed in the switched-on state in a manner dependent on different control signals, i.e. can be set to different negative output voltages. The influence of negative substrate bias voltages of different magnitudes on the functionality of the semiconductor chip can thereby be tested.
Control signals are required for setting the respectively desired output voltage of the substrate bias voltage generator, the control signals being fed to the semiconductor chip during test operation. In order to achieve a high integration density of the components on the chip and hence a small chip area, as few control signals as possible should be necessary in order to be able to set as many operating states of the voltage generator as possible.
It is accordingly an object of the invention to provide a circuit configuration for generating a controllable output voltage usable for example as a substrate bias voltage of an integrated circuit that overcomes the above-mentioned disadvantages of the prior art devices of this general type, which requires the least possible area consumption in an integrated realization.
With the foregoing and other objects in view there is provided, in accordance with the invention, a circuit configuration for generating a controllable output voltage. The circuit configuration contains a voltage generator having input terminals receiving a supply voltage, an output terminal providing the controllable output voltage and, a control terminal controlling the controllable output voltage. A comparator is provided and has inputs receiving a reference signal and a comparator control signal derived from the controllable output voltage and an output connected to the control terminal of the voltage generator. Supply terminals are provided for receiving the supply voltage to be forwarded to the voltage generator. Transistors are provide and have control terminals and controlled paths receiving the supply voltage to be forwarded to the voltage generator. A first of the transistors is connected between a first of the supply terminals and one of the input terminals of the voltage generator, and a second of the transistors is connected between a second of the supply terminals and another of the input terminals of the voltage generator. A logic device is connected to the control terminals of the transistors and outputs a transistor control signal for driving the transistors. A switching network is connected to the comparator and receives at least a first control signal and a second control signal. A level of the comparator control signal derived from the controllable output voltage is dependent on the first and second control signals.
In the case of the invention, control signals are fed on the one hand to the switching network for setting the magnitude of the output voltage, and, on the other hand, the same control signals are fed to a logic device, which serves, through the driving of corresponding transistors, for switching off the supply voltage that can be fed to the voltage generator. As a result, all possible state combinations of the control signals are used to set the magnitude of the output voltage, including the switching-off of the voltage generator. Consequently, only a few control signals are required and, accordingly, few signal lines for providing the control signals are required. The area requirement in an integrated realization is therefore limited to the most essential amount.
The logic device contains a corresponding logic switching device which generates a control signal that causes the supply voltage of the voltage generator to be switched off when a single predetermined combination of signal states of the control signals is present at the logic device.
The logic device contains a gate for performing a logic operation. One of the control signals is fed to the gate of the logic device and also to the switching network in non-inverted form. The rest of the control signals are fed to the gate of the logic device in inverted form, but to the switching network in non-inverted form, or respectively vice versa. The gate is preferably one that carries out a NOT-AND combination (NAND gate).
The switching network preferably contains a series circuit having a number of resistors corresponding to the number of control signals. Switches are respectively disposed in parallel with the resistors, which switches can be controlled by a respective one of the control signals. The switches contain, for example, two transistors of complementary conductivity types, whose controlled paths are connected in parallel and whose control terminals are controlled by complementary control signals derived from the respective control signal.
For an as linear as possible control of the output voltage in a manner dependent on the control signals, it is provided that the resistances of the resistors are proportional to one another, i.e. the resistances of the resistors differ from one another by a constant multiplicative factor. In particular, the resistances of the resistors may be a sequence of powers of two of a basic resistance.
The voltage generator is a regulator that generates the output voltage from the supply voltage fed to it. In particular, the voltage generator generates a voltage lying outside the supply voltage that is fed. If the supply voltage is positive, e.g. has a value of +2.5 volts with respect to reference-ground potential, then the substrate bias voltage is in the negative direction relative to reference-ground potential and has a value of approximately xe2x88x920.7 volt. As is known such a voltage pump operates in a clocked fashion. As a result of the clocked-controlled charging and charge-reversal of capacitances within the voltage pump, the negative substrate bias voltage is generated from the positive supply voltage. The magnitude of the output voltage is regulated by the clocked operation of the voltage generator being switched on and off and thus being kept within a specific bandwidth.
The control signals cause a suitable number of resistors of the switching network to be short-circuited. The control signal for the comparator, which is derived from the generated output voltage, e.g. the substrate bias voltage, is shifted in a predetermined manner by this measure, so that the voltage generator accordingly supplies a different output voltage. Overall, through a predetermined combination of signal states of the control signals, either the voltage generator can be completely switched off or the magnitude of its output voltage can be set accordingly.
The resultant setting possibility for the magnitude of the generated output voltage, e.g. substrate bias voltage, is important particularly during test operation in order to check the semiconductor chip for functionality and operational reliability at different substrate bias voltages. During test operation, the control signals are input into the chip by a digital control word, buffer-stored and forwarded to the logic device and the switching network. The semiconductor chip can be put into the test mode only after a very particular command input, which is not present in normal operation. Thus, although the capability of setting the output voltage is afforded in test operation, it is switched off in normal operation. During normal operation the voltage generator is always supplied from the supply voltage and the switches of the switching network have a fixedly predetermined, invariable switching state.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a circuit configuration for generating a controllable output voltage, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.